Hermetically sealed containers such as encapsulated electrical devices and specialized electrochemical cells and the like are ordinarly subjected to leakage testing to evaluate the effectiveness of the hermetic seal. Generally, to leak test such a device, it is placed in a pressurized atmosphere of a test gas, such as helium, for a period of time to force the test gas into the hermetic container through the hermetic seal. No hermetic seal is perfect. Thus, some gas will always enter the container. Helium is preferred as the test gas since it has an extremely small molecule relative to most leakage opening sizes.
The test is completed by examining the hermetic container to determine if any of the helium, which entered it, is escaping. For example, one standard test for leakage is the submersion test in which the container is submerged in a heated fluorocarbon liquid, as described in Mil-STD-202E and Mil-STD-883. The formation of bubbles while submerged indicates the escape of the test gas.
In many cases such a test is gross and more refined techniques are required. Such a refined technique is provided by the use of a mass spectrometer which draws a vacuum on the hermetic container and provides an indication of the amount of test gas removed from the container.
Unfortunately, to perform these test procedures, the hermetic container is exposed between the time it is in the pressurized test gas and placement of the container in the mass spectrometer apparatus. During this exposure, even for very brief periods, a substantial amount of the test gas, particularly if it is helium, can be lost from the device, undetected. In very short times, if the volume for accepting the test gas inside the hermetic container is small, all of the test gas may be lost before the mass spectrometer can perform its test. In such an instance, the mass spectrometer indicates no test gas present and it can only be inferred that there is no leak or that the leak is so gross that all of the test gas has been lost.
It is the purpose of this invention to provide a seal arrangement which always delays the loss of test gas, thus allowing adequate time for testing the container in a mass spectrometer following exposure of the hermetic container to the pressurized test gas and which is more practical from a production standpoint.